It's possible that it's incoming hits from Search Engines; well crawled content that's been linked to shows higher in search requests through them...

And theres a simple way to tell, look at the Who's Online page, a bot will have a "user agent" string that tells you where it's from, and it'll show in the "guests" or "bots" section (it'll show in the bots section if you tell your ubb which UA's are bots)

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